What should I do if stray cats keep getting into the backyard?

There's 3 of them and they all look alike like they are siblings and I guess they get in my climbing the fence. Is there any kind of repellent that wouldn't harm them?

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NodandaWink · 51-55, M

Cayenne pepper. Sprinkle a hefty amount around where they come over the fence or where they generally pass through. They will snuff it up and get the hot pepper in their nose. One time and they wont be back. It does no damage but they really won't like the memory of the incident.
I have done this with great effect.